rec.autos.simulators

Engine Braking : One for the physics guys I think

Maxx

Engine Braking : One for the physics guys I think

by Maxx » Fri, 02 Feb 2001 02:03:32

There a bunch of clever guys in this newsgroup, so I though
I'd pose a question that's been bothering me.

OK, first thing, what do "I" mean by engine braking.

Essentially I mean using the engine, via the gearbox, diff, drivetrain
to exert a braking force on the rear wheels (I know it can be front
wheels but we will just consider a RWD car).

We accomplish this by changing down to a lower gear and
re-engaging the clutch at a higher speed than the gear and current
revs would equate to. Thus if we were travelling at 170mph in 5G,
at 9,500 revs and we changed down to 4G we would get engine
braking (and perhaps breaking!).

We wouldn't normally do the above of course, the correct way
to USE engine braking is to brake and change down to a lower
gear at aorund the speed you would be going at max revs in
the lower gear, thus you get good engine braking but shouldn't
damage the engine and shouldn't lock the wheels.

I've used engine braking, in real-life and GPL. I know there are
current F1 drivers using engine braking. The thing is, does it
really help?

Surely if we are braking optimally already, the tires will not accept
any more braking force than we are giving them.

I know engine braking was used in the past as brakes in those
days where simply not powerful enough to give maximum braking
force, that generally is not true of today cars/brakes.

Also, I know that under braking the weight balance of the car
shifts forward considerably. We'd generally set the brake bias
to compensate for that, but we may be able to alter this weight
transfer (as was discussed in gas+brake posts) by using engine
braking, allowing the rear (wider) wheels to come into play more.

Also of course, having engine braking doing some of the slowing
down will alleviate some of the load from the brake pads which
at under very heavy braking will reach very high temperatures at which
they are less efficient.

All that said is there ANY DIFFERENCE, in physics terms between
brakes->pads->disk->hub->wheel->tire than engine->drivetrain->
axle->hub-wheel->tire.

The sort of thing I'm thinking off is, we know that adding power
under braking will negate some of the weight being thrown forward
as we brake.  Would braking via the axle lead to a difference in
weight transfer than conventional braking.

Is there any other reason, other than those mentioned above
why engine braking might be more efefctive than conventional
braking?

TIA

Maxx

Art McEwe

Engine Braking : One for the physics guys I think

by Art McEwe » Fri, 02 Feb 2001 02:20:47

One thing engine braking would do would be to effectively change your brake
balance rearward for a second.  This could be good and bad..

I know my down habitual shift points from the Honda resulted in me putting
the Brabham backwards into the hedging at parabolica..

jbo..

Engine Braking : One for the physics guys I think

by jbo.. » Fri, 02 Feb 2001 02:43:12

This doesn't really answer your question, but I've heard many racers
and mechanics state that brakes and brake pads are cheaper than
transmissions and engines.  When racers use heel-and-toe
clutch/brake/throttle techniques to match the engine's RPMs to the the
speed of the drivetrain/wheels, this effectively removes engine braking
from the equation all together.

-- JB
   F1CS2K Rank 68.354
   http://f1cs2krank.dhs.org



<SNIP>

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Maxx

Engine Braking : One for the physics guys I think

by Maxx » Fri, 02 Feb 2001 03:05:55


>This doesn't really answer your question, but I've heard many racers
>and mechanics state that brakes and brake pads are cheaper than
>transmissions and engines.  When racers use heel-and-toe
>clutch/brake/throttle techniques to match the engine's RPMs to the the
>speed of the drivetrain/wheels, this effectively removes engine braking
>from the equation all together.

No, Heel & Toe removes BAD engine braking from the equation,
hence damaged engines and transmission. H&T is actually a
requirement of correct engine braking.

Bad engine braking is when you drop it into the lower gear at low
revs (as you've let the engine rev down under braking) and it will
create all sorts of stresses in the engine and  transmission, also
likely lock the wheels.

Good engine braking is using H&T to match the engine revs
AND changing down at what would be the speed = Max Revs
in the gear you are changing to.

This way you get the same level of engine braking you get
when you just lift off at Max Revs (which is more than
if you lift off at lower revs).

GPL effectively does H&T for us. But by changing down later
(say at a speed equal to 1/2 of max revs in the target gear)
you are probably getting some small amount of engine push
as the revs are still dropping from max revs (blip) and may
well be over 1/2 max revs when the gear is engaged. It's
not terribly significant in itself as an "aqnti-braking" force, but
it will unsettle the car a little.

I'm glad you raised the point however as it allows me to
explain what I'm meaning in a little more detail.

Maxx

GraDe

Engine Braking : One for the physics guys I think

by GraDe » Fri, 02 Feb 2001 03:42:57

My theory on engine braking....

It's all pants:-)
I hate comparing racing to road cars but in this instance I will.
If you are driving your road car simply try engine braking and then not
doing it...
You usually use engine braking over a longer stretch of road like coming to
a junction at the end of a dual carriageway.
You use brakes to stop you  quickly.

Yes engine braking works, but I cannot see how it can aid you to brake
faster.
The only way I can brake very efficiently in GPL is to stay in a high gear
until I have a very low speed for the next corner then shift, overwise the
momentum in the engine is carrying me forward.

As I say you can use engine braking is real life, I just believe personally
that it's not beneficial in racing... I'm sure someone will try to prove
this wrong though, i have no solid evidence only my interpretation.

bryde..

Engine Braking : One for the physics guys I think

by bryde.. » Fri, 02 Feb 2001 04:18:32

Maxx,

I'm no physicist, but I clearly remember Carroll Smith dealing with
this topic in one of his " ... to Win" books; probably Drive to Win.
His basic point after a long preamble about car physics was something
like... "engines are for acceleration, brakes are for braking...". I'd
have to sift through it to provide details. I'm not sure about 1967,
but in modern racing it makes sense to allow the brakes to handle all
the decceleration (unless there is some wierd turn which requires
heavier rear braking (bias) to maintain speed and control compared to
the rest of the track).

But as has been mentioned brakes are cheaper to replace; a point also
made by Smith I believe.

A side note about your question of the physics of how the slowing down
is accomplished: in the late 70's or early 80's Ferraris had inboard
rear brakes, near the gearbox. The half-shafts would carry all the
acceleration and braking stresses to the wheels. I think the key
advantage was the reduced "unsprung" weight of the wheel assembly. I
suspect they dropped the design due to cooling issues or broken half-
shafts. Or the higher weight of reinforced half-shafts.

Cheers,

Ray

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Leo Landma

Engine Braking : One for the physics guys I think

by Leo Landma » Fri, 02 Feb 2001 06:51:21



Look at it from the tyre contact patch up: it doesn't matter what causes the
slow down, or where it comes from - it all starts before the bearing (I hope
this makes sense) so the effect on grip will be the same.
The effect on car behaviour of course is very different <g> It seems I can't
drive without engine braking anymore (in GPL that is).

Bye,
Leo

J. Todd Wass

Engine Braking : One for the physics guys I think

by J. Todd Wass » Fri, 02 Feb 2001 09:45:47

  Breaking, yes!  lol

  Exactly.  If your brakes can lock all four tires already, engine braking is
pointless, unless you are trying to decrease brake wear.

  >I know engine braking was used in the past as brakes in those

  Right.  Once the brakes are strong enough, there's no point to engine braking
in regard to slowing the car more rapidly.  The car will only slow down as
quickly as the tires allow.

  What really ends up controlling weight transfer in the end is the vehicle's
acceleration.  Whether the acceleration is caused by the brakes, the engine, or
both at the same time shouldn't matter.  The end "g" is all that counts.

   >Also of course, having engine braking doing some of the slowing

  Right.  

  No difference whatsoever.  

  I know you've all been on that subject for a long time, so much so that I've
not read nearly all the posts, but from what I've been able to gather, the only
reason adding some throttle will decrease the amount of weight thrown forward
is because we're reducing the vehicle's acceleration by hitting the gas.  The
car's weight transfer is determined solely from one thing:  Acceleration.  Of
course, you claim from experience that it does indeed happen anyway, so I won't
argue with you about that :0)  You might be getting some transient effects that
are different from steady state deceleration weight transfer stuff.  I haven't
raced anything but go-karts for $13.50 for 8 minutes at a time, so there may be
something I'm not thinking of here.

  In the end, the only thing that effects steady-state weight transfer is
acceleration.  By steady-state, I mean once the weight has all transferred, and
you're decelerating at a constant rate.  (Not the first instant when you hit
the brakes, that's a transient condition.)  Whether this is coming from a
combination of brakes and throttle, or either of these seperately, one
acceleration/deceleration should always result in a constant weight transfer:

  Load transfer = Acceleration (g) * (Weight * cg_height / wheelbase)

  The acceleration is determined by force via:
  Acceleration = Force / Mass

  To find the acceleration, and thus, the load transfer, I'd add together the
forward/rearward forces at all four tires.  If you've got 1000 lbs of braking
force at all four tires, and 200 lbs of throttle input at the rear (800 lbs net
for each rear tire), you should end up with 3600 lbs of force.  Let go of the
throttle, and you should have 4000 lbs of force (more weight transfer, and more
deceleration if the rears don't lock.)  Divide that by the mass, and you've got
the acceleration.  Divide that by 32.16 to get the acceleration in g's.  Then,
run this result through the first equation as (g) to get the load transfer.

  Your experience seems to indicate otherwise, but unless there's something I'm
missing here (quite possible), load transfer is really only influenced by
acceleration.  

    >Would braking via the axle lead to a difference in

  Nope.  Steady-state weight transfer is only influenced by acceleration,
regardless of what force or combination of forces cause it.

  Not that I can think of.  If you've got enough braking power to lock the rear
tires, adding more torque by including engine braking won't help.  The only use
I can think of for engine braking is, as you mentioned, taking some load off
the brake pads to reduce heat and wear.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

J. Todd Wass

Engine Braking : One for the physics guys I think

by J. Todd Wass » Fri, 02 Feb 2001 09:58:48

  There are two different things here that people need to look at (not picking
on you, this is for everybody :0) ).  One is actual engine braking, caused by
low pressure in the cylinders.  This will exist when the throttle is closed
regardless of whether heel-toe downshifting is used or not.

  Then what's the point of heel-toe downshifting?  Because of the other effect:
 A reaction torque.  When we heel-toe downshift, we're trying to get the engine
speed to land exactly where it should be (as we all already know).  In other
words, if the clutch is spinning at 5000 rpm's, we want the engine to be
spinning at 5000 rpm's when we let off the clutch.  If we let the engine idle,
then let off the clutch, we all know that we get a big lurch forward and
sometimes the rear wheels lock/break loose when combined with some real
braking.  

  This effect is seperate from engine braking.  The friction at the clutch
causes the engine/flywheel and the clutch disk itself to try and match speeds.
The result is, in a downshifting scenario, the engine speeds up and the
clutch/drivetrain/wheels slow down.  There's an additional torque or "reaction"
sent through both ends during that short period of time when the clutch disk
and flywheel are not locked together.  Once they are locked together, the
effect is done, but the engine, running at high rpm now and with no throttle,
is still running very low cylinder pressure, causing a "negative" torque to run
through the drivetrain, acting *exactly* like a brake.  This is what I consider
to be engine braking.

  So engine braking isn't removed from the equation when heel-toe downshifting
is used, however, the reaction torque, which is usually several times larger
than any actual engine braking torque, *IS* removed from the equation, as you
said :0)  Just wanted to make sure folks know there are two different issues
here.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

J. Todd Wass

Engine Braking : One for the physics guys I think

by J. Todd Wass » Fri, 02 Feb 2001 10:02:22

  I agree with you here.  I think what racers are talking about when they are
"engine braking" is operating under "trailing throttle."  

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

Ale

Engine Braking : One for the physics guys I think

by Ale » Fri, 02 Feb 2001 10:30:21

How does the differential work when you use engine
to brake? Doesn't it help to distribute braking
power between inside and outside wheels?

Btw, another unrelated observation. While driving
FWD car on the snow using throttle and brakes
at the same time has some benefits. It allows
to lock rear wheels before front, and turn the
car nicely. Same as using handbrake, but more control.

Alex
(alexti)



Greg Campbel

Engine Braking : One for the physics guys I think

by Greg Campbel » Fri, 02 Feb 2001 14:29:12


>....

> I've used engine braking, in real-life and GPL. I know there are
> current F1 drivers using engine braking. The thing is, does it
> really help?

FWIW, I use it in GPL.
On tracks that like lots of trail braking or braking when going over
hills, I set the brake bias a few clicks further forward than usual.
This makes the car more drivable and less likely to bite back.  With the
unbalanced (inefficient) bias, straight line braking suffers.  To
reclaim it, I just downshift a bit earlier than usual.  It makes for a
noticeable improvement in stopping power.

I'd think so.

Does this count as e.b.? - Use the engine to power an oversized,
reversed radiator fan for aero braking!

If If the brakes are overheating, they are poorly designed/configured or
something had broken.

I can think of one issue:  During engine braking, the drive train
channels torque back into the engine.  In a longitudinally mounted
motor, this may cause chassis flex and other undesirable reactions.

No.  Weight transfer is proportional to deceleration.  It doesn't matter
where that wheel does the braking.  There may be some 'extra' forces
involved, but I don't think they will add up to anything worth worrying
over.  Adding power just reduces your deceleration and, therefore,
subsequent w. transfer.

-Greg

Race15

Engine Braking : One for the physics guys I think

by Race15 » Fri, 02 Feb 2001 14:30:44

"engines are for acceleration, brakes are for braking..."

I remember reading this also, although for the life of me I can't remember
where.

Mike

Tony Whitle

Engine Braking : One for the physics guys I think

by Tony Whitle » Fri, 02 Feb 2001 16:44:41


It's not unrelated, the opposite is true for a RWD car - engine braking
allows more braking to be done by the rear tyres, leaving the front tyres to
be used for steering. Think of the traction circle.

--
Tony Whitley
GPLRank +10.65

Ville V Sinkk

Engine Braking : One for the physics guys I think

by Ville V Sinkk » Fri, 02 Feb 2001 18:19:45

<snip>

<snip>

<snip>

*What*? I could never have imagined that the matching of downshifts
with the engine revs could have anything at all to do with car
balance! I've always been downshifting at as low revs as possible in
places where you need balance and 100% control of the car (like Nouveau
Monde and Mexico T1), because it feels safe. Eg. Approaching Nouveau
Monde at G4 I start braking normally, but only start shifting down when
I'm halfway through the last bit of straight that leads to the hairpin
itself.

So, if I'm not entirely mistaken, what you're saying is that I should do
exactly the *opposite* to keep the car in control? Please elaborate further...

+Cinquo [GPLRank 30.19]
+Cinquo


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